The New York Times has an interactive chart (screen-grab above) showing how rarely budget forecasts correspond with reality. Ezra:
The dark line is the deficit, while the silver threads shooting are forecasts. As you can see, what forecasters do is extend the current trends. When something happens to break that trend -- a massive financial crisis, say -- they're generally caught unawares. Which is as you'd expect. If they knew a crash was coming, they'd be making money betting against the market, not running budget models.
And the reverse is also true as well. Beyond a certain point, you can get surprised on the upside as well. And we still don't know the future of this recovery for sure.